The invention relates to a discharge lamp designed for dielectrically impeded discharges. Such a discharge lamp typically has a discharge vessel, which contains the discharge medium, conventionally xenon, or more generally a gas mixture with a noble gas. For ignition and maintenance of the discharges, electrodes are provided; discharge lamps designed for dielectrically impeded discharges are distinguished in that at least the electrodes designed as anodes are separated from the discharge medium by a dielectric layer, which can also be a wall of the discharge vessel. It is also possible for all the electrodes to be dielectrically impeded, for instance to make the discharge lamp suitable for a bipolar electrical power supply.
The fundamental physical events, technical properties and advantages as well as possible applications in or of discharge lamps for dielectrically impeded discharges are understood here to be known. The relevant literature can be referred to.
One essential performance characteristic for discharge lamps designed for dielectrically impeded discharges, that is, so-called xe2x80x9csilent xe2x80x9d discharge lamps, is the temporal and local homogeneity of the luminance. Special provisions for varying the distribution of the discharge in the discharge space have this as the goal, in particular individual localizable discharge structures, by means of special electrode structures that form preferential points for the discharge structures. Reference can be made for instance to German Patent Application DE 196 36 965 A1. By localizing the single discharges by means of electrode structures, optimized patterns in terms of the three-dimensional arrangement can be specified, which fill the discharge space in such a way that a favorable distribution of the luminance occurs. However, there is still a need for improvements to the temporal and local homogeneity of the luminance, above all in discharge lamps with significant length in at least one direction, such as barlike lamps with one direction of longitudinal extension, and flat lamps with two such directions.
The invention is thus based on the technical problem of disclosing a silent discharge lamp that is improved in terms of the temporal and local homogeneity of the luminance.
According to the invention, this problem is solved by a discharge lamp for dielectrically impeded discharges, having a discharge vessel filled with a discharge medium and having discharge electrodes which are at least partly separated from the discharge-medium by a dielectric layer, wherein the discharge vessel is elongated at least in a longitudinal direction, characterized by a thermal device for controlling the heat transport into and out of the lamp nonhomogeneously in the longitudinal direction, which is designed such that in operation, the temperature in the lamp is made homogeneous in the longitudinal direction.
The invention proceeds from the recognition that some nonhomogeneities in elongated silent discharge lamps occur only after time has elapsed in operation. It was possible to conclude that there was a relationship between the homogeneity of the temperature distribution in the discharge vessel and the homogeneity of the light projection. Evidently, since a homogeneously equally distributed pressure prevails in a discharge vessel, a nonhomogeneity in the temperature distribution results in a nonhomogeneous distribution of density of the discharge medium. The density of the discharge medium in turn has an effect on the physics of the discharge. In this sense, evidently a nonhomogeneity in the temperature resulting from the installation situation, the construction of the lamp itself, external temperature nonhomogeneities or other causes, is the cause of fluctuations in luminance over the at least one longitudinal direction in which the lamp extends.
In particular, it has been found that these variations in luminance can develop over the operating time, or in other words are generally linked via the purely local variation to a variation over time in the luminance distribution in the initial phase of operation. Thus the lack of temperature homogeneity is disadvantageous in two respects.
The general inventive concept is in the most general sense to exert influence on the temperature distribution by means of a thermal device, which controls the heat transport into and/or out of the lamp. According to the invention, this involves not simply a powerful cooling device, for instance, with which in a sense the attempt is made, by means of a suitably generous design of the cooling apparatus, to impress its temperature homogeneity on the lamps. Instead, the point of departure for the invention is that the thermal device in turn nonhomogeneously influences the heat transport, specifically in a way that is complementary to the intrinsic temperature behavior of the lamp. This is intended to counteract the development of the nonhomogeneous temperature profile in the discharge lamp.
In principle, the term xe2x80x9cthermal device xe2x80x9d used here covers any provisions by which influence can be exerted on the heat transport. In particular, it includes the control of the heat transport to the outside out of the lamp and in the opposite direction. Accordingly cooling devices in the most general sense can be considered, that is, devices that reinforce and improve heat dissipation from the discharge lamp to the outside, insulating devices, that is, devices that reduce the heat transport, which in general means heat transport from the discharge lamp to the outside, and finally also heating devices.
In very many cases, the intrinsic temperature behavior of the lamp, that is, the nonhomogeneous temperature profile that occurs without the thermal device of the invention, is characterized in that peripheral regions of the discharge lamp are not heated as much during operation as middle regions. This can be due for instance to the fact that the peripheral regions, referred to the portion of the discharge space assigned to them, have a larger surface area and thus greater heat losses. However, the invention also pertains to cases that are otherwise, for instance in which because of special mounting situations, the closeness of other components that produce heat, special discharge vessel geometries or otherwise, nonhomogeneous temperature profiles occur.
Concrete possibilities for thermal devices of the invention are for instance cooling bodies with cooling fins that are in thermal contact with the discharge lamp; the presence of the cooling fins, their length, or the density of their disposition is nonhomogeneous in a way that is adapted to the intrinsic temperature behavior of the discharge lamp. For instance, the cooling fins may either be present only in the middle of the lamp, or be located closer together in the middle of the lamp, or be stretched out with a larger surface area. Nonhomogeneous cooling can also occur from a mounting device which in the middle region of the discharge lamp is coupled with good thermal conductivity, for instance, and acts as a cooling device by means of its own good thermal conductivity. This may for instance be a solid metal body. Naturally, both of these provisions may also be combined.
Another possibility is to insulate the peripheral regions of a discharge lamp thermally from the outside world, or else, by making thicker insulators or other components with insulating properties that are present anyway, such as discharge vessel walls, to provide for reinforced insulation in the peripheral region. For details of these various possibilities, see the exemplary embodiments in the further course of this description.
In the introductory part of the description, conventional provisions have already been mentioned with which influence can be exerted on the three-dimensional distribution of individual discharge structures. What is essential is that the provisions proposed with this invention and these conventional possibilities do not in any way preclude one another but instead prove to reinforce one another. In this sense, the invention is directed in particular to discharge lamps designed for the pulsed operating process developed by the present Applicant. This pulsed operating process assures the development of localizable individual discharge structures. For details, reference may be made to the prior art, and in particular to International Patent Disclosure WO94/23442. In particular, the invention is thus also directed to a discharge lamp, designed according to the invention, with a ballast device provided for the pulsed operating process.
One important application of silent discharge lamps is discharge lamps with an elongated barlike discharge vessel. In other words, they are elongated in only one longitudinal direction, and in the plane perpendicular to it are relatively small in cross section. Important applications of such xe2x80x9clinear radiators xe2x80x9d are in the field of office automation (OA), for instance. They can be used in scanners, such as in fax machines, electronic copiers, or in computer peripherals. They are equally suitable for conventional photocopiers. In this respect, it should be stated that the invention relates not only to discharge lamps that produce visible light but to UV radiators, for instance, as well.
In the field of these linear radiators, the invention is especially advantageous in relatively powerful linear radiators, in which experience tells that the disadvantages that are overcome or at least ameliorated by the invention occur to an increased extent. Powerful linear radiators can for instance have linear power densities of over 0.3 W/cm.
However, the invention is equally suitable for use in flat radiators, that is, large-area, essentially two-dimensionally extended discharge lamps, for instance for lighting liquid crystal screens from behind. In such flat radiators as well, greater cooling of a middle region relative to a peripheral region, or better insulation of the peripheral region from the middle region, or heating of the peripheral region is advantageous, that is, a thermal device along the lines of the invention. In principle, the possibilities already described can be chosen, such as cooling fins; the cooling fins are disposed correspondingly nonhomogeneously not only in the longitudinal direction but also in the transverse direction (that is, in the plane of the flat radiator). One example of this is a component part of the exemplary embodiments that will be described hereinafter.
Another possibility that is also illustrated in the exemplary embodiments has a generally flat metal sheet, which is in superficial thermal contact with the discharge vessel of the flat radiator. Recesses are provided in the metal sheet and define ribs that divide at least a middle region of the sheet from a peripheral region, and optionally also define a plurality of intermediate regions graduated from the middle region toward the peripheral region. The middle region of the metal sheet can be cooled by being embodied as a cooling device itself, for instance with cooling fins, or by being in thermal contact with a cooling device. The ribs make it possible to vary the heat transport from the peripheral region into the cooled middle region, so that once again, nonhomogeneous control of the heat transport out of the lamp into the metal sheet can be effected. The directly cooled middle region of the sheet will in fact cool the lamp more markedly than the outer region or regions joined to it only via the ribs.